A present-day mass-market television receives and displays 25 images per second, each made up of two frames, one containing the even lines and the other the odd lines of the image; the screen is thus scanned vertically 50 times a second, this being a low frequency in relation to the reaction time of the eye. The limit frequency above which the eye would succeed in temporally resolving a televized sequence into a succession of discrete images is not much less than 50 Hz. By contrast, this flicker is virtually eliminated for a frame frequency of 75 Hz or better of 100 Hz; this latter solution therefore corresponds to a doubling of frequency.
The most obvious solution for achieving this frequency doubling consists simply in repeating twice on display each frame received; this is why such a device is termed 100 Hz AABB. It is also an interlaced display (the 312.5 even lines are displayed for 10 ms and then the 312.5 odd lines for the following 10 ms, etc.).
The shortcomings of such a simplistic algorithm quickly become apparent however; effective on stationary images, it proves to be tiring to the eyes if an object is shifting on the screen. A horizontally moving point is seen as if double, all the more so the greater its speed; this is due to the fact that it is observed by the eye along with the successive images as occupying one position twice and then a neighbouring position twice, etc (see FIG. 1). The systems for controlling the eye enabling it to follow a shifting object are highly disturbed by these irregularities, and do not allow it to identify the original motion clearly.
A good solution consists in detecting and calculating the motion of the various parts of the image and in calculating the intermediate images with the help of this information; this is the strategy which has been adopted here. The main defect of the 100 Hz AABB described above is thus corrected while preserving the suppression of flicker.
However, another defect of present-day television sets is linked with the interlacing of the lines displayed. In an image having good definition, a fine detail such as a line on the screen will, owing to the interlacing, only be displayed once every 1/25 of a second (see FIG. 2). A solution consists in displaying all the lines of the screen with each vertical scan, while reconstructing the non-transmitted lines by temporal or spatial interpolation. It could then be asked why not display 625 lines for each frame, with a renewal frequency of 100 Hz.
If the frame frequency of 100 Hz is retained, this amounts to again doubling the number of lines displayed per second, and hence to quadrupling the frequency of scanning the lines of the screen as compared with a conventional receiver: this is known as working at line frequency 4 H.
In a cathode-ray tube, the deflexion of the electron beam providing the scanning of the spot is controlled by a system of electromagnets situated at the base of the tube. For so high a scanning frequency, the bulk of the energy absorbed by these electromagnets is restored in the form of heat, this fairly quickly rendering them unsuitable for their role in the current state of mass-market technology. It is therefore necessary to reduce the line frequency.
The upper limit presently achieved is 3 H. We are therefore led to consider a standard based on a frame frequency of 75 Hz, with 625 lines per frame. The flicker effect is almost invisible at 75 Hz; the screens of certain workstations, for example, have a frame frequency of 72 Hz, this rendering them much less tiring to look at than a television screen.
The invention proposes a process for interpolating progressive frames from interlaced frames.
European Patent Applications 95 400722.5 (D94/128I) and 95 400721.7 (D941128I) in the name of Thomson Consumer Electronics SA describe processes and devices for converting images from a 50 Hz interlaced format to a 100 Hz interlaced format.
European Patent Application EP-A-064047 also in the name of Thomson Consumer Electronics relates to a method of measuring confidence in a motion estimate in an image conversion system.